My personal opinion is that if you you want to drop less than $100 you should just forget about it in the first place unless you can find something used..
A $50 point and shoot camera is not worth the effort, much less so if you want it housed for that money..
---------- Post added January 6th, 2014 at 12:48 AM ----------
A VERY quick summary of what you really NEED although you might not think you want it from a low budget UW camera is;
Manual or at minimum semi-auto modes, not only fully automatic.
RAW format capability - Yes, it does make a huge difference on the image quality.
Internal flash (as you wont be spending the money for strobes, much less hotshoed strobes and housing electronics for it).
Unless you have these three things you are likely to be disappointed and soon to replace the system or just give up on uw photography.
Then of course theres the optics and electronics with resulting image quality of the cheapest p&s systems...